You asked: Why distinct is bad in SQL?

The fact that the resultset has duplicates is frequently (though not always) the result of a poor database design, an ineffective query, or both. In any case, issuing the query without the DISTINCT keyword yields more rows than expected or needed so the keyword is employed to limit what is returned to the user.

Is it bad to use distinct in SQL?

If you’re querying a table that is expected to have repeated values of some field or combination of fields, and you’re reporting a list of the values or combinations of values (and not performing any aggregations on them), then DISTINCT is the most sensible thing to use.

Is distinct bad for performance?

However, in more complex cases, DISTINCT can end up doing more work. Essentially, DISTINCT collects all of the rows, including any expressions that need to be evaluated, and then tosses out duplicates. GROUP BY can (again, in some cases) filter out the duplicate rows before performing any of that work.


Why is distinct slow?

If the query already involves an active ORDER BY that requires a non-passive sort, the SELECT DISTINCT may end up being basically “free” (and may even be faster as fewer rows may be returned), but this situation is rather rare, and just adding “DISTINCT” to a query that doesn’t do an ORDER BY will make it run at least …

Is distinct costly in SQL?

In a table with million records, SQL Count Distinct might cause performance issues because a distinct count operator is a costly operator in the actual execution plan. … You can replace SQL COUNT DISTINCT with the keyword Approx_Count_distinct to use this function from SQL Server 2019.

Does distinct slow down a query?

Running with the DISTINCT keyword

If you do, your phone will ring, your pager will go vibrate, your users will have a hard time forgiving you, and performance will slow to a crawl for a little while. A quick examination of the query plan reveals that a table scan is still being used to retrieve the data from the table.

What we can use instead of distinct in SQL?

Using GROUP BY instead of DISTINCT | SQL Studies.

Should I use Groupby or distinct?

If you want to group your results, use GROUP BY, if you just want a unique list of a specific column, use DISTINCT. This will give your database a chance to optimise the query for your needs. Please don’t use GROUP BY when you mean DISTINCT, even if they happen to work the same.

Is it better to use distinct or GROUP BY?

To make your code easier to understand, you should use distinct to eliminate duplicate rows and group by to apply aggregate operators (sum, count, max, …). Doesn’t matter, it results in the same execution plan.

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How do I make distinct faster?

3 Answers

  1. SELECT DISTINCT is slower than expected on my table in PostgreSQL.
  2. Select first row in each GROUP BY group?
  3. Optimize GROUP BY query to retrieve latest row per user.

Why select distinct is bad?

As a general rule, SELECT DISTINCT incurs a fair amount of overhead for the query. Hence, you should avoid it or use it sparingly. The idea of generating duplicate rows using JOIN just to remove them with SELECT DISTINCT is rather reminiscent of Sisyphus pushing a rock up a hill, only to have it roll back down again.

Is distinct fast?

DISTINCT creates a temporary table and uses it for storing duplicates. GROUP BY does the same, but sortes the distinct results afterwards. is faster, if you don’t have an index on profession . All of the answers above are correct, for the case of DISTINCT on a single column vs GROUP BY on a single column.

Is GROUP BY faster than distinct SQL Server?

DISTINCT is used to filter unique records out of all records in the table. It removes the duplicate rows. SELECT DISTINCT will always be the same, or faster than a GROUP BY.

How do I count distinct rows in SQL?

The COUNT DISTINCT function returns the number of unique values in the column or expression, as the following example shows. SELECT COUNT (DISTINCT item_num) FROM items; If the COUNT DISTINCT function encounters NULL values, it ignores them unless every value in the specified column is NULL.

How do I count the number of distinct rows in SQL?

To count the number of different values that are stored in a given column, you simply need to designate the column you pass in to the COUNT function as DISTINCT . When given a column, COUNT returns the number of values in that column. Combining this with DISTINCT returns only the number of unique (and non-NULL) values.

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How can I get distinct values in SQL JOIN?

You can use CTE to get the distinct values of the second table, and then join that with the first table. You also need to get the distinct values based on LastName column. You do this with a Row_Number() partitioned by the LastName, and sorted by the FirstName.

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